In order to reduce the density of a sheet molding compound (SMC) or bulk molding compound (BMC), high-density inorganic fillers, such as calcium carbonate, are removed from the formulation and replaced with thermoplastic in an effort to maintain surface quality characteristics. Another approach common to the field is to remove a portion of the inorganic filler and replace some of the inorganic filler with hollow glass micro spheres. U.S. Pat. No. 7,700,670 is exemplary of this effort. Owing to the tendency of such glass microspheres to have limited bonding to the hydrophobic surrounding matrix, this reference teaches modifying the surface of the glass micro sphere with a silane to create stronger bonding between the micro spheres and the cured matrix in which the microspheres are encompassed. This greater interaction is observed as higher sheer strength and the microspheres being encrusted in matrix material upon failure in micrographs. While the resulting cured material have been amenable to creating a surface with a high quality surface finish with specular gloss measurement of 70 gloss units (GU), it has proved difficult to pass industry durability tests for paint adhesion required of automotive exterior paints.
While the industry has contemplated articles formed of SMC with specific densities of less than 1.5, as evidenced by U.S. Pat. No. 5,412,003 in order to achieve weight reduction in articles. This has practically proven difficult as high loadings of microspheres leads to a degradation in the handling properties of the pre-cured resin and mechanical properties. One persistent problem with low density SMC articles is poor overcoating adhesion thereby making painting of the article.
Thus, there exists a need for an ultra low-density SMC or BMC article containing hollow glass microspheres that has strong inherent adherence to an over layer.